I download Secondary Structure Files from https://www.rcsb.org/pdb/static.do?p=download/http/index.html#ss. I want to know what does it mean the letter after PDB code in the secondary structure(header)?
>102L:A:secstr HHHHHHHHH EEEEEE TTS EEEETTEEEESSS TTTHHHHHHHHHHTS TTB HHHHHHHHHHHHHHH HHHHHH TTHHHHHHHS HHHHHHHHHHHHHHHHHHHHT HHHHHHHHTT HHHHHHHHHSSHHHHHSHHHHHHH HHHHHHSSSGGG
I mean the letter 'A' in this case. Thanks.